Apparatus and method to test embedded thermoelectric devices

ABSTRACT

An integrated circuit containing an embedded resistor in close proximity to an embedded thermoelectric device. An integrated circuit containing an embedded resistor in close proximity to an embedded thermoelectric device composed of thermoelectric elements and at least one switch to disconnect at least one thermoelectric element from the thermoelectric device. Methods for testing embedded thermoelectric devices.

This application is a continuation-in-part of application Ser. No.12/790,688 filed May 28, 2010, which application Ser. No. 12/790,688 isa continuation-in-part of application Ser. No. 12/201,679 filed Aug. 29,2008 and also claims priority from and the benefit of ProvisionalApplication Nos. 61/182,052 filed May 28, 2009 and 61/182,055 filed May28, 2009; which application Ser. No. 12/201,679 claims priority from andthe benefit of Application No. 60/968,805 filed Aug. 29, 2007; theentireties of all of which are incorporated herein by reference.

BACKGROUND

This relates to the field of integrated circuits. More particularly,this relates to apparatus and methods for the testing of integratedcircuits containing embedded thermoelectric devices.

Integrated circuits with embedded thermoelectric device are known anddescribed in U.S. patent application Ser. No. 12/201,679. After themanufacturing of an integrated circuit containing an embeddedthermoelectric device is complete, the integrated circuit and theembedded thermoelectric device are tested. The chips with integratedcircuits that fail to meet specifications and with thermoelectricdevices that fail to meet specifications may be scrapped or sent forfailure analysis.

Typically, the integrated circuit is tested using conventional finaltest equipment and the thermoelectric device is tested using specialequipment which may apply a temperature gradient across the integratedcircuit chip while an electrical output is monitored. The specialtesting equipment and additional test time required to evaluate thethermoelectric device adds additional cost and cycle time. In addition,it is difficult to achieve accurate thermal coupling betweenthermoelectric testing equipment and the device under test, so it can bedifficult to achieve reproducible and accurate testing using anexternally applied temperature gradient.

A thermoelectric device may be composed of many thousands ofthermocouples that may be coupled together serially and in parallel.Typically, when a thermoelectric device fails to meet specifications itis scrapped or sent to failure analysis. It may be challenging and timeconsuming during failure analysis to locate the one or several badthermocouples out of thousands or tens of thousands of goodthermocouples. Furthermore, as the number of thermocouples increases, itbecomes increasingly difficult to find defective thermocouples bytesting the entire array as a unit.

SUMMARY

An embedded resistor heater/thermometer is formed in close proximity tothe thermoelectric device. A method to test an embedded electricaldevice using conventional electrical final test equipment. A methodusing electrical switches contained in a thermoelectric device toidentify which thermocouple or thermocouples caused an embeddedthermoelectric device to fail specifications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E are cross-sectional and top views of an integrated circuitwith an embedded thermoelectric device and an embedded resistoraccording to principles of the invention.

FIG. 2 is a circuit diagram of the embedded thermoelectric device withan embedded resistor according to principles of the invention

FIG. 3 is a thermoelectric device containing switches that may be testedaccording to principles of the invention.

FIG. 4 is a thermoelectric device containing switches that may be testedaccording to principles of the invention.

FIG. 5 is a graph illustrating the temperature dependence of thesubthreshold slope of a nmos transistor.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

By fabricating an embedded resistor near an integrated thermoelectricdevice, conventional final test equipment may be used to evaluate thefunctionality and performance of the thermoelectric device. The embeddedresistor may be used as a resistance heater to cause a thermal gradientto form across the thermoelectric device or it may be used to monitor achange in temperature of the chip caused by operation of the embeddedthermoelectric device. This may be performed using conventional finaltest equipment and may eliminate the need for special equipment andsignificantly reduce the time needed to test an embedded thermoelectricdevice thus significantly reducing cost.

The term embedded resistance heater/thermometer refers to an embeddedresistor that is constructed in close proximity to the thermoelectricdevice and that may be used as a resistance heater or may be calibratedto monitor chip temperature. The embedded resistor may have two or moreterminals. A four terminal resistor allows accurate measurement ofresistance because two terminals may be used to force current from acurrent source, while the other two terminals connect to a voltmeter tomonitor voltage. This allows resistance to be measured accuratelyregardless of contact resistance at the terminals. The current sourcemay be a current mirror, a bandgap-referenced current source, or otheron-chip current reference. The voltmeter may be an analog-to-digitalconverter. The embedded resistor may also ye configured to connect to anexternal current source in an off-chip tester or in another element ofthe system in which the integrated circuit is incorporated.

In some circumstances it may be possible to use a two-terminal embeddedresistor instead of a four-terminal embedded resistor. For example, thecontact resistance in the interconnects may be highly repeatable, thusallowing for precise calibration of the resistance measurement. Incertain integrated circuits where it is to be used as a heater onluy,and embedded two terminal resistance heater my be used because thevoltage need not be sensed.

An example embodiment embedded resistance heater/thermometer 134 that isformed in close proximity to a thermoelectric device is illustrated inFIG. 1. FIG. 1A through 1D are a cross section 100 describing thethermoelectric device 101 and a resistance heater/thermometer 136. FIG.2 is a circuit diagram 150 corresponding to the cross section 100. Likeelements are labeled with the same number in cross section 100 andcircuit diagram 150. The integrated circuit is constructed in a p-typesubstrate 102 and includes shallow trench isolation (STI) 112, anisolated pwell 108 contained in a deep nwell 104, an nwell 146, a p-typethermopile 106 and an n-type thermopile 144 connected together by ametal-1 strap 138. The two thermopiles, 106 and 144, form a thermocouple101. The thermocouple may be used in a thermogeneration mode to generateelectricity or in a thermo refrigeration mode to provide cooling. Theintegrated circuit also includes premetal dielectric (PMD) 114, contacts110, intermetal dielectric (IMD1) 116, metal-1 118, IMD2 122, metal-2136, IMD-3 124, via-2 126, and metal-3 128. Although the thermoelectricdevice 101 consists of one thermocouple in FIG. 1, an embeddedthermoelectric device may contain many thousands of thermocouplesconnected in series and in parallel.

The layout of Metal-3 leads illustrated in a top down view in FIG. 1Cand FIG. 1D, forms a 4-terminal Kelvin resistor with metal-2 resistor136. Current may be supplied to resistor 136 through leads 128 in FIG.1C and 132 in FIG. 1D. The voltage developed across the resistor 136 maybe measure using leads 130 in FIG. 1C and 134 in FIG. 1D. The resistancemay then be calculated with the equation, R_(R)=V_(R)/I_(R). The changein resistance may be calibrated and used to monitor the temperature ofthe chip. The resistor 136 may also function as a resistance heater byforcing current through the resistor 136 to supply a heat source forembedded thermoelectric generators.

The current generated by embedded thermoelectric device 101 may bemeasured at the output leads 118 and 140. Similar to the resistor 136, afour terminal measurement may be performed across the thermoelectricdevice 101 using the leads shown in the top down views in FIG. 1B andFIG. 1E. For example, when the thermocouple is being used in athermocooling mode, current may be supplied through leads 118 in FIG. 1Band 140 in FIG. 1E. The voltage that is produced across the thermocouple101 may be measured using leads 120 in FIG. 1B and 142 in FIG. 1E. Theresistance of the thermocouple may be calculated with the equationR_(TE)=V_(TE)/I_(TE).

A number of functional tests may be performed on the thermoelectricdevice 101 with conventional final probe equipment using the embeddedresistor, 136, either as a resistance heater or as a chip temperaturemonitor.

In one embodiment test may be to measure the open circuit voltage,V_(OC), produced by thermogenerator, 101, as a function of the amount ofcurrent forced through resistor 136. V_(OC) may be measured across leads120 and 142 with leads 118 and 140 disconnected from Vcc and Vss. Aseries of currents, I_(R), may be forced through the resistance heater,136, to generate a series of thermal gradients across thethermogenerator, 101. A curve of V_(OC) vs I_(R), may be constructed andcompared to specifications. This test may be performed with conventionalfinal probe equipment.

In another embodiment test the short circuit current, I_(SC), producedby thermogenerator, 101, may be measured. In this test a series ofcurrents, I_(R), may be forced through resistance heater, 136, to createa series of different temperature gradients across the thermogenerator,101. At each of the temperature gradients, a current, I_(SC), may beforced through leads 118 and 140 in FIG. 1F, until the voltage, V_(TC),across leads 120 and 144 is equal to zero. The current at this conditionis the short circuit current, I_(SC), current. A curve of I_(SC) vsI_(R), may be constructed and compared to specifications. This test maybe performed with conventional final probe equipment.

Resistor 136 may be calibrated to monitor chip temperature by forcing aseries of currents, I_(R), through the resistor, 136, and measuring thevoltage, V_(R), across the resistor, 136. The voltage may be fit to acubic equation, V_(R)=C₀+C₁I_(R)+C₂I_(R) ²+C₃I_(R) ³, where C₀, C₁, andC₃ are curve fitting constants. Points from this graph may be pluggedinto equation, V_(R)/I_(R)=R_(R,T0) (1+TC_(R)I_(R) ²R_(R,T0))/Θ_(R),where C_(R) is the temperature coefficient of resistance for theresistor 136 material to determine the value of, Θ_(R), which is thethermal impedance of resistor 136. The temperature may then becalculated using the equation, T=T₀+(I_(R) ²R_(R,T0))/Θ_(R). The valuefor C_(R) may be taken from a table if the resistor, 136, is made from apure metal or may be determined by measuring resistance vs temperatureby placing an integrated circuit chip with the embedded resistor, 136,in a bake oven and taking resistance measurements at severaltemperatures. The embedded resistor, 136, may be calibrated in thismanner and the data from this calibration may be used for all subsequentchips with the same integrated circuit manufactured with the sameprocess.

The resistance across thermoelectric device 101 may be calibrated in amanner similar to that described for the embedded resistor, 136, byforcing a series of currents, I_(TC), across leads 118 in FIG. 1B and140 in FIG. 1E and measuring the voltage drop, V_(TC), between leads 120in FIG. 1B and 142 in FIG. 1E.

In addition to calibration of the resistor, 136, or the thermodevice,101, in FIG. 1 as chip temperature monitors, other integrated circuitcomponents such as a silicon diode, or a transistor subthreshold slopemay be measured at various temperatures and used to evaluate theperformance of a thermoelectric device. For example, the forward voltageof a silicon diode is temperature dependent. By comparing the bandgapvoltages, ΔV_(BG), of a silicon diode at two different currents, I₁ andI₂, the temperature may be determined using the equation,ΔV_(BG)=(kT/q)ln(I₁/I₂). An electric circuit, such as the Brokaw bandgapreference may be used to measure ΔV_(BG) and therefore monitor thetemperature for various embedded thermoelectric device operatingconditions.

FIG. 5 shows the change in an nmos transistor subthreshold slope (SS)versus temperature. The subthreshold slope gets less steep as thetemperature of the transistor changes from 25 C for SS trace 504 to 85 Cfor SS trace 502. These means of monitoring temperature of an integratedcircuit chip may also be utilized with conventional final probeequipment to test functionality of embedded thermoelectric devices.

Another embodiment method to evaluate performance of a thermoelectricdevice, 101, may be to run the thermoelectric device in a refrigerationmode and to monitor the temperature of the integrated circuit chip. Inthis mode a series of currents, I_(cooling) may be forced between pads140 and 118 in FIG. 1F. At each, I_(cooling), the temperature of theintegrated circuit chip may be monitored by measuring the resistance,R_(R), of resistor, 136, the resistance, R_(TC), of thermoelectricdevice, 101, the change in transistor subthreshold slope or the bandgapof a silicon diode. The change in resistance, subthreshold slope,bandgap voltage or temperature may be compared to specifications todetermine if the thermoelectric cooler is functioning properly. Thesetests may also be performed using conventional final probe equipment.

The above example embodiment functional tests may be performed to testan embedded thermoelectric device using conventional final testingequipment. Other functional tests, such as generating a thermoelectricdevice load line may also be performed and is within the scope of thisinvention.

A pulsed method of testing may also be advantageous to determine theheat capacity of the integrated circuit and to determine the rate atwhich the integrated circuit heats up and cools down. The heat capacityof an object is equal to the specific heat capacity, C_(v), times thevolume, V. The equation, −KΔT=C_(v)Vd(ΔT)/dt, where K is the thermalconductance of the integrated circuit, ΔT is difference in temperatureof the integrated circuit and the surroundings, and d(ΔT)/dt is thechange in the temperature delta with respect to time, may be integratedto give ln(ΔT)=−tK/C_(v)V+c, where c is a constant. A log plot of ΔT vst has a slope of K/C_(v)V which is a characteristic of the thermalconductance and heat capacity of the integrated circuit.

Providing pulses of current to the embedded resistor heater may also beused to measure the generated current, I_(sc), and/or the generatedvoltage, V_(oc), as a function of time. The time delay between theresistor temperature and the generated current or voltage may beinterpreted in terms of heat capacity and thermal conductance betweenthe resistance heater and the thermoelectric device.

If an embedded thermoelectric device fails specifications it may be verydifficult to locate a few defective thermocouples among thousands ofthermocouples. As the number of thermocouples increases, it becomesincreasingly difficult to find defective thermocouples by testing theentire array as a unit, so a test method enabling testing ofthermocouples individually or in groups smaller than the entire array isdesirable. Such a test method is essential for high yield and highquality in volume production of integrated circuits incorporating one ormore embedded thermoelectric devices. Determining the location ofdefective thermocouples in a large array of thermocouples may be greatlyenhanced by incorporating switches in the thermoelectric device as shownin FIGS. 2A and 2B. Testing smaller portions of a thermoelectric arrayincreases the sensitivity and may allow the detection of defects thatare not detectable by array-wide testing. For example, one badthermocouple has much more impact on an array of 10 ten thermocouplesthan on an array of ten thousand thermocouples. This increased abilityto detect and locate defective thermocouples may be important forproduct reliability

The example thermoelectric device, 300, in FIG. 3 consists of 4 columnsof thermoelectric elements 302 (thermoelectric generators orthermoelectric coolers) connected in series. Each of the thermoelectricelements, 302, may be an individual thermocouple or an array ofthermocouples. In this example embodiment each column may be connectedor disconnected from the thermoelectric device, 300, using a switch,304. If there is a short in one of the columns causing thethermoelectric device, 300, to fail, the failing column may beidentified when it is disconnected and the thermoelectric device thenfunctions properly. Switches, 304, may be transistors or pass gates.Each of the functionality tests described in above embodiments may beperformed on the thermoelectric device, 300, with one or more of thecolumns disabled. Other tests such as diode leakage and tests for shortsand opens may also be conducted and columns causing excessive leakage oropens identified.

Similarly thermoelectric device, 406, in FIG. 4 consists of 3 rows ofthermoelectric elements 408 (thermoelectric coolers or thermoelectricgenerators) connected in parallel. Each of the thermoelectric coolers,408, may be an individual thermocouple or an array of thermocouples. Inthis example embodiment each row may be connected or disconnected fromthe thermoelectric device, 406, using a switch, 410. Switches, 410, maybe transistors or pass gates that are formed when the thermoelectricdevices are formed with no additional patterning or processing steps. Ifthere is a short or open in one of the rows causing the thermoelectricdevice, 406, to fail specification, the failing row may be identifiedwhen it is disconnected and the thermoelectric device functionsproperly.

Those skilled in the art to which this invention relates will appreciatethat many other embodiments and variations are possible within the scopeof the claimed invention.

What is claimed is:
 1. An integrated circuit, comprising: an embeddedthermoelectric device; and an embedded resistor in close proximity tothe embedded thermoelectric device.
 2. The integrated circuit of claim1, wherein the embedded resistor is a formed in an interconnect layerover the embedded thermoelectric device.
 3. The integrated circuit ofclaim 1, wherein the embedded resistor is a four terminal resistor. 4.The integrated circuit of claim 1, wherein a four terminal resistor isformed with the embedded thermoelectric device as a resistance element.5. The integrated circuit of claim 1, wherein the embeddedthermoelectric device contains an array of thermoelectric elements withat least one switch that may disconnect at least one of thethermoelectric elements from the embedded thermoelectric device.
 6. Anintegrated circuit, comprising: an embedded thermoelectric devicecomprised of an array of thermoelectric elements; a switch capable ofdisconnecting at least one of the thermoelectric elements from thethermoelectric device; and an embedded resistor in close proximity tothe embedded thermoelectric device.
 7. The integrated circuit of claim6, wherein the embedded resistor is formed in an interconnect layer overthe embedded thermoelectric device.
 8. The integrated circuit of claim6, wherein the resistor is a four terminal resistor.
 9. The integratedcircuit of claim 6 where a four terminal resistor is formed with theembedded thermoelectric device as a resistance element.
 10. A method oftesting an integrated circuit, comprising: forcing a least one currentlevel through an embedded resistance heater in close proximity to anembedded thermoelectric generator; and measuring an output from thethermoelectric generator at each of the current levels.
 11. The methodof claim 10, wherein the output is a voltage.
 12. The method of claim11, wherein the voltage is an open circuit voltage.
 13. The method ofclaim 10, wherein the output is a current.
 14. The method of claim 13,wherein the current is a short circuit current.
 15. The method of claim10, further comprising: performing a first test of the embeddedthermoelectric generator by forcing a current through the embeddedresistance heater and measuring a first output; disconnecting athermoelectric element from the thermoelectric generator; performing asecond test of the embedded thermoelectric device by forcing the currentthrough the embedded resistance heater and measuring a second output;and comparing the first output to the second output.
 16. The method ofclaim 10, wherein the step of measuring is a steady state measurement.17. The method of claim 10, further comprising: forcing the at least onecurrent level through the resistor for a fixed period of time; andmeasuring the output versus time after the fixed period of time.
 18. Amethod of testing an integrated circuit, comprising: forcing a currentthrough an embedded thermoelectric device causing it to perform as arefrigerator; monitoring an electrically measured variable that changeswith the temperature of the embedded thermoelectric device.
 19. Themethod of claim 18, wherein the electrically measured variable is aresistance an embedded resistor.
 20. The method of claim 19, furthercomprising calibrating the embedded resistor by forcing a series ofcurrent levels through the embedded resistor, measuring a voltage acrossthe embedded resistor at each of the current levels and fitting anequation where the voltage is a dependent variable, current is anindependent variable and where the equation is a cubic equation.
 21. Themethod of claim 18, wherein the electrically measured variable is aresistance of embedded the thermoelectric device.
 22. The method ofclaim 18, wherein the electrically measured variable is a subthresholdslope of a transistor.
 23. The method of claim 18, wherein theelectrically measured variable is a bandgap voltage of a silicon diode.24. The method of claim 18, wherein the monitoring step is a steadystate measurement.
 25. The method of claim 18, further comprising:forcing the current for a fixed period of time; and performing themonitoring step versus time after the fixed period of time.
 26. Themethod of claim 17, further comprising: performing a first test of theembedded thermoelectric device forcing a current through an embeddedthermoelectric device and recording a first electrically measuredvariable; disconnecting a thermoelectric element from the thermoelectricgenerator; performing a second test of the embedded thermoelectricdevice by forcing the current through the embedded resistance device andmeasuring a second electrically measured variable; and comparing thefirst electrically measured variable to the second electrically measuredvariable.